3.2mm nozzles and 1.8 bar... Would expect the combination to flow way too much gas.

Without re-reading the whole thread, can you see petrol rail pressure in OBD live data? If so: When running on petrol what is frp? / when running on gas what is frp? If there is a big difference in figures (e.g. frp rises by more than say 20%) then you will need to fit a fuel return if you haven't already done so (and before you'll get anything like good results).

I am currently converting a Merc C65AMG 6.5 litre V12 twin turbo, that has been tuned to maybe 800bhp with a wider stainless exhaust, increased boost, etc. But I don't expect to run as high flowing a combination of pressure and nozzles. This model has a return-less petrol system and does read frp, so while I was running Faro between the tanks and the reducers I also ran a petrol pipe from the engine bay back to the petrol tank.. Will see if I need to actually plumb it in but it seems very likely.

I just re-read and noticed you said spuds have only 3.1mm hole - Remove those spuds and fit spuds with bigger hole! As you know, the spuds are the restriction now, but this means your sequential system will become a continuous injection system at higher loads, which will really mess things up.

I expect the OMVL system has been wired incorrectly to petrol injector 1. Do the figures for inj 1 fluctuate as we'd expect (in fact do any/all of them fluctuate)? Do cyl 1 figures read properly when running on petrol? Does the car misfire on cyl 1 on petrol / on gas?

Run a pipeline from that pump over here!

Simon

Hi Simon,

My apologies for not replying before now. Trying to fit too much in as usual...

I've sort of thrown my toys out of the pram with the LPG - or more to the point, got fed up as to why I can't find this flatspot/stutter as it otherwise ran brilliantly on LPG. I'd been meaning to drop the pressure again as per your recommendation, but it really did run well on the configuration as set. I can't increase the 'spuds', as I am running on injector risers. Actually, the toy throwing was more to the fact the one of the reducers failed at the Le Mans Classic a few weeks back, and it dumped 3.7 bar to the injectors. It took a little while to try and work out why pipes kept popping off. I drove the 700km back on petrol, parked the thing in the garage at work in Germany, and haven't touched it since. We are taking the old Dodge on holiday next week for 5 weeks, so I'll hopefully be motivated to get back on it when we return.As for the petrol pressure, yes it rises when running on LPG, but may I ask why this causes a problem? Does the engine compensate somehow when the pressure rises?

Have you done a write-up anywhere on the S65 AMG? I would certainly love to read about that and see some pics...

Builder Brad wrote:

Hey Piffle,

nice install on the AMG! I registered on this site so I could post this....

Ive done something similar in my BMW 550i, but wanted to increase the range per fill from what I had on petrol. Managed to get a 120 Litre 4 hole tank in the boot after quite a bit of faffing about.

I have a couple/3 of questions as I think that the only improvement over what I have may be with an AMG.lol. there are a lot of other alternatives that would be better for me, but not viable as the engines are direct injection. natural progression for me would be an M5 as I now have deacent knowlege around car.

is your car the M156 engine?

also, what range do you get on average per l of LPG?

what width and depth is availible in the boot for a large cylinder 4 hole tank? dont know how you put up with a donut tank, as your mpg has got to put you permanantly planning your next fuel stop.

cheers

Brad

Hi Brad, Yes, it's an M156 engine. It does about 7km per litre on gas when on a run, a heck of a lot less when driving spiritedly! I am running a tiny tank - 51 litre - it will take about 42 litres from empty. The reason for this is that the car has 7 seats, so the rear seats protrude quite significantly into the wheel well. It's a 650x200 tank if I remember correctly. Without the third row of seats, you could easily double the height. Of course you could fit a huge cylinder tank, but then that would completely defeat the point in having an estate. I reckon it would be a good option in a saloon, as the boot space is really deep.I fill up every 250km ish, but I don't begrudge that on the UK-Germany commute, as it is currently about 26 pence per litre in Belgium.

The bit about the petrol pressure is relevant - put as simply as possible the petrol injector timing will be adjusted relative to fuel pressure, so will be shorter if its higher pressure to give the same amount of petrol being injected. The next question is likely to be how much it rises, and the answer may be a fuel return to the tank (to allow dumping excess back to tank to maintain the pressure at a fairly constant level.

The issue being that the lpg pressure doesn't rise in relation to the petrol pressure, so the lpg system can't make an accurate adjusted injector timings as depending on what the petrol pressure is alters the injector duration timings. The pressure will also be higher without a return on petrol when running on lpg as the petrol isn't getting squirted out of the injectors either.

Simon will probably/hopefully give you more detail, but I think he is on holiday at the moment from other posts he has made, so it may be a few days before he can do so.

Agreed with Brian. If pressure rises to a higher than normal level and stays at that peak it will need a fuel return. Both the C65AMG (V12bi turbo) and ML63AMG (6.3V8) I recently converted needed fuel returns - If one is needed and you don't fit one you could potentially damage your petrol pump, would never be able to calibrate properly. The ML63AMG I converted has the same engine and is the same model year as yours!

The electronic substitutes (for physical fuel returns) don't address the issue of potential damage to the petrol pump and seldom work properly (by work properly they should have the petrol ECU reading an emulated fuel pressure that is very close to what the pressure would be under driving on petrol conditions). For the Merc, I don't believe a simple plug n play electronic substitute is available anyway, a programmable version might work but would need programming so you'd need to buy the programming gadget too. Better and probably cheaper just to fit a physical return...

I didn't take any pics but I'll be seeing the car again soon when the owner comes to collect his other AMG ML that I currently have in the yard.

His other AMG ML is the older ML55AMG which doesn't need a fuel return, he arrived in it to collect the ML63AMG and left it with me for servicing and fettling... The previous owner was an LPG installer, had pulled out most of the stops during conversion including fitting a smaller petrol tank to allow fitting of an LPG tank in that location, modified the exhaust to fit an LPG tank underslung at the rear, so 2 LPG tanks. Fettling was necessary because shortly after buying the car drive-ability and MIL light problems started - To cut a long story short it was in need of proper calibration.

I second the request to see pictures of that glorious v12 engine with one of your conversions.

There was a certain irony this weekend as you were just talking about over pressuring, and I was thinking of you as I lay under my van isolating the dead fuel pump... We were starting our summer holidays on Saturday in our beloved '83 Dodge Ram camper. After 11 hours waiting for a ferry from Dover (which is actually fine when you have a fridge and toilet on board!), we were driving past Antwerp, when I noticed some smoke in the rear view mirror. Then LOTS of smoke! The engine spluttered and stalled as I came to a stop. Balls.Seems the mechanical fuel pump had failed (although the van does have a fuel return ) and pumped the 10 or so litres of petrol from the tank directly into the sump. The smoke was the resultant oil/petrol mix spraying out of the engine directly onto the exhaust. Gulp.A quick roadside oil change/isolation of the fuel pump, and we were on our way again. Thank goodness the van runs on LPG!

Anyway, the e63 will remain unused in Germany until the beginning of September. I've bought a couple of Magic3 repair kits, so I hope I can fix it enough to go back down to a sensible pressure before attempting to drive the car home. Perhaps I'll then have some enthusiasm back to install a return feed so I don't kill another fuel pump!

Bad news regards the petrol pump but at least you could continue to run on LPG . Unusual for a mechanical petrol pump on a carb'd engine to be damaged without a fuel return fitted though?

Reducers can be damaged if there isn't a good enough liquid feed from the tank(s). With a single tank there might always be the problem of not a good enough feed for sustained flat out driving (totally on gas) with a 500bhp+ engine.

Unusual for a mechanical petrol pump on a carb'd engine to be damaged without a fuel return fitted though?

Some of the more modern ones had a return but the older ones didn't. The lever pushed the diaphragm to suck fuel in and it was pumped to the carb by a spring on the other side of the diaphragm so if the float valve was shut, no fuel went anywhere.

Many years ago a mate of my brothers bought a VW Beetle to me that had a strange fault. You could start it up and it ran fine for the first few seconds then it would start to make strange banging noises and the dipstick would leap out of it's tube. That had a split diaphragm in the fuel pump so was dribbling petrol into the sump, the wear in the bores meant there was enough blow by on the rings so it was igniting the fuel vapour in the crankcases and firing on both sides of the pistons......

Know what you mean regards some having returns. I was kind of implying that if there is no return fitted it should be the spring operated type and therefore not be damaged without a return fitted, while if it has a return there shouldn't be need to fit another...

Last year I converted (yet) another Talbot 2L camper, the previous ones I'd converted (including one I owned) didn't have a return fitted, this one did have a return which seemed to have been fitted aftermarket. Both myself and the customer noted before LPG conversion it could be a bugger to start on petrol but I had no problems starting it on petrol (or LPG) during the latter part of my time with it so I just put it down to using a different start procedure (choke/throttle/etc). Few weeks later the owner called to say she couldn't start it. I had already given her printed documentation on how to start on petrol / start on LPG / fuel changeover methods / etc, so I told her to follow the instructions for starting on LPG and she managed to start it. She could then switch to petrol after starting on LPG, was supposed to be visiting at some point for me to look at the return but hasn't been back yet. I reckon the problem might be that with petrol in the carb it will start on petrol (so won't start after running on LPG with empty carb float bowl), because with the aftermarket return fitted there isn't enough pressure/flow to lift fuel into the carb during cranking. If I found this to be the case I'd probably be fitting a restriction into the return.

Reads like an engine rebuild will have been in order for the Beetle... Could have been prevented if they'd brought the car to you sooner.

Nah, the Beetle got a fuel pump and an oil change and that was it. Back to being the horrible, underpowered, pile of sh*te it always had been. There's some cars that you either love or hate and the Beetle is one of those (the original Mini being the other) that I absolutely detest but others rave about.

I suspect the problem on the Ram was the same as the Beetle, the diaphragm split from old age rather than anything else.

Nah, the Beetle got a fuel pump and an oil change and that was it. Back to being the horrible, underpowered, pile of sh*te it always had been. There's some cars that you either love or hate and the Beetle is one of those (the original Mini being the other) that I absolutely detest but others rave about.

I suspect the problem on the Ram was the same as the Beetle, the diaphragm split from old age rather than anything else.

Totally with you on the Beetles (and anything flat 4 air cooled). Hateful things!

The Dodge does actually have a factory return pipe. It is a single in/out on the pump, but the output then goes into a small filter can with an output to the carb, and a smaller output to the return pipe. Doesn't appear to be a regulator anywhere. This is definitely as original because I just ordered a new filter.But yeah, I assume that pump to be the original, 33 years old and 205000km, so probably just a split diaphragm. Just ordered a new one for €30, so an easy/cheap fix. I converted it to LPG nearly 10 years ago, and it's not skipped a beat since. It starts from cold on LPG, and we've only ever had to switch it to petrol max 3 times since.I dread to think how long that petrol had been in the tank, and it was probably no use at all as a fuel. However, as an engine flush, I believe it just performed sterlingly!

What I used to do on older cars with mechanical fuel pumps was make up a blanking plate to block off the hole where they would fit and then use a modern electric pump. You can, or could, get an electronically controlled replacement for the old clunk, clunk, clunk SU pumps fitted to some BL and Jaguar carb equipped cars and they work really well.

[quote="piffle"]Oh, and cheapest LPG found to date - that's about 26pence per litre! :D :D :D

I can beat that.

Total station in Ghent this morning, my 8th fill up on the way back from Saulkrasti, a small seaside town about 30 miles north of Riga in Latvia. Due to the Antwerp ring road being blocked by an accident, my satnav sent me round the top and past Ghent but by this time I was well out of my comfort zone looking at the trip meter. It was showing 213.8 miles since the last fill in Germany and I'd been sitting in traffic too so expected it to be virtually empty. Empty takes 67 litres to fill so I has plenty in reserve.

Apologies for no updates in the last half year... My contract ended in Germany, so I came back to Blighty and got a role locally. I'm ashamed to say that the e63 has been parked up around the side of the barn and sort of forgotten about. At least it avoided the salt this year!

Thought I'd better get it going again. It's still got this hesitation problem on LPG that Simon was almost certainly right in saying that I need a fuel return.

Do you think by emulating the fuel pressure, it would solve the hesitation problem? I'm guessing this wouldn't actually reduce the petrol pressure though, so would the pump would be still be under strain? Could I get away with this rather than running another pipe?

I've converted a few AMG63 (and an AMG65 bi turbo V12) and all in the AMG 6X range that I've done have needed a fuel return... but it is possible that some of the early models might not need a return. Way to tell, usually, is to check if there is a petrol pressure sensor on the fuel rail or a petrol pressure reading in OBD live data (if so, then you need one).

I don't usually even attempt to fit an electronic emulator, because I don't find they usually work, at least not long term without bringing on the MIL due usually to ECU seeing lack of pressure fluctuations as a problem, and also because there is potential for damaging the petrol pump.

I did recently fit an electronic emulator on an old Ford F150, but this vehicle had previously run on LPG for years and tens of thousands of miles without damaging the petrol pump before I fitted the electronic emulator, so it had already been proven that lack of return wasn't going to damage the pump on that model. The F150 ECU doesn't interpret unusual lack of petrol pressure fluctuation as a problem but I think the Merc ECU will.

A generic adjustable type of electronic emulator with simple adjuster screw (pot / variable resistor) worked fine on the Ford, but don't know if the same part would work on the Merc (or as said above, maybe not in the long term). Other types of petrol pressure emulator do return a slight fluctuation pressure reading but they're not usually adjustable with a pot, and since the Merc runs unusally high pressure it may be the case that a purpose designed type would have to be fitted (for higher pressure reading and fluctuating reading).. and even then, again, will it damage the fuel pump or pump driving electronics..?

Fitting an emulator or fuel return won't be a full fix, you'll also need to recalibrate the LPG system from scratch.

If you fit a physical return, do yourself a favour... fit pipe that will withstand the high fuel pressure and flush pipe and fittings through thoroughly with petrol before piping in - The Merc injectors and fuel rail don't even have the usual last ditch gauze type filters.

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